1. Field of the Invention
The present invention is related to an annealing separator used in the finishing annealing step of the producing process of a grain-oriented electrical steel sheet, and to a finishing annealing method for producing a grain-oriented electrical steel sheet. Particularly, the present invention is related to an annealing separator which satisfies conditions of both a stabilization of the secondary recrystallization in the finishing annealing and an improvement of the forsterite insulating film.
2. Description of the Related Arts
The conventional grain-oriented electrical steel sheet is a 0.10-0.35 mm thick steel sheet containing, usually, up to 4.5% by weight of Si, which is constituted, on the entire surface, by crystal grains having a (110)[001] orientation to the rolling direction (Goss oriented grains). The surface of the grain-oriented electrical steel sheet is usually covered with forsterite (Mg.sub.2 SiO.sub.4), to ensure achievement of the insulative property. That is, the grain-oriented electrical steel sheet is a composite material composed of a silicon-containing electrical steel sheet having an extremely highly oriented (110)[001] texture (Goss texture) and a surfacial ceramic material of oxide-series, i.e., the forsterite. The surfacial ceramic material is thin, i.e., from 0.1 .mu.m to a few microns in thickness.
An extremely high orientated Goss texture and a thin forsterite insulating film are heterogeneous to one another from the view point of material science. The achievement of such heterogeneous material is performed during a current production process for the grain-oriented electrical steel sheet, virtually simultaneously, in the single step of final annealing within an annealing box. To enhance the orientation of the Goss texture, a catastrophic grain growth (abnormal grain growth), referred to as secondary recrystallization, is industrially utilized. On the other hand, the forsterite film is formed by a solid-state reaction between the SiO.sub.2, which is contained in the oxide film preliminarily formed on the surface of a steel sheet, and MgO, which is a major component of the annealing separator applied to the oxide layer. The secondary recrystallization and the forsterite formation are in essence, fundamentally different from one another. They are, however, liable to be equally influenced by the annealing atmosphere and any additive to the annealing separator, which is composed mainly of magnesia. The secondary recrystallization and the forsterite formation presumably proceed actually under a mutual interference therebetween at the interface of the interior of a steel sheet and surfacial part. From this point of view, much research has been carried out up to the present, regarding the annealing atmosphere and additives of the magnesia.
The weight of coils subjected to the final annealing is constantly increasing, towards an enhancement of the productivity. This inevitably leads to an enhancement of the temperature- and gas atmosphere-distribution along the length or width of the coils, and hence, to an enhancement of the nonhomogenity of the coil interior. The various compounds added to the annealing separator are useful for keeping the nonhomogenity of the coil interior to as low a level as possible. This has been a driving force behind the research into the additive compounds.
The additives of the annealing separator have two widely classified effects: A stabilization of the secondary recrystallization, and a stable formation of a forsterite film. In the former case, selection of the kind of additives is based on the criterion of which material mechanism induces the secondary recrystallization. In this regard, the presence of fine precipitates, referred to as the inhibitor, is indispensable for the secondary recrystallization; since the secondary recrystallization is usually stabilized by strengthening and maintaining the precipitation phases of the inhibitor up to a high temperature, and measures for stabilizing the secondary recrystallization are usually taken to ensure, in the annealing atmosphere, an adequate nitrogen partial pressure for the inhibitors composed mainly of nitrides and an adequate sulfur partial pressure for the inhibitors composed mainly of sulfides.
First, the prior art methods of ensuring an adequate nitrogen partial pressure are described. Japanese Examined Patent Publication No. 46-937 discloses that the annealing of an Al-containing silicon steel sheet in the nitrogen atmosphere is useful. This method was further developed as the method of Japanese Examined Patent Publication No. 46-40855, according to which various annealing methods of an Al, Ti, Zr, or V containing silicon steel are disclosed. Japanese Examined Patent Publication No. 49-6455 points out the utility of selectively nitriding the surface layer of an Al-containing silicon steel sheet. Japanese Examined Patent Publication No. 54-19850 proposes to adjust the dew point of the finishing annealing-atmosphere within a range of from -20.degree. C. to +30.degree. C., to attain an adequate absorption of nitrogen in the annealing atmosphere by the steel sheet. Japanese Examined Patent Publication No. 54-22408 proposes to carry out the finishing annealing in a nitrogen atmosphere containing 20% or less of hydrogen. Further, a metal nitride, specifically chromium nitride, titanium nitride, or vanadium nitride, is proposed in Japanese Examined Patent Publication No. 54-14568 as an additive for annealing separator, which allegedly lessens the dispersion of the annealing atmosphere along the width and length of a coil, by homogenizing the nitrogen partial pressure of the annealing atmosphere.
Second, the prior art methods of ensuring an adequate sulfur partial pressure are described. In this connection, Japanese Unexamined Patent Publication No. 53-50008 proposes, for the purpose of stabilizing the secondary recrystallization of silicon steels, in which the inhibitors composed mainly of Sb and S and/Se are utilized, to add a sulfur compound, such as Fe.sub.2 S, to the annealing separator or to carry out the finishing-annealing in a gas atmosphere containing H.sub.2 S.
As is apparent from the above description, the secondary recrystallization tends to be stabilizied by controlling the nitrogen or sulfur partial pressure in the finishing annealing. The additives of the annealing separator are used for stabilizing the nitrogen or sulfur partial pressure. The additives of the annealing separator are also used for stably forming a forsterite film. In order to facilitate the progress of solid state reaction between the MgO applied to the surface of a steel sheet and the SiO.sub.2 preliminarily formed thereon, which results in the formation of forsterite (Mg.sub.2 SiO.sub.4) as described above, an additive material having a catalystic action is usually advisable. For example, MnO.sub.2 and TiO.sub.2 are disclosed as the additives in Japanese Examined Patent Publication No. 51-12450, and Japanese Examined Patent Publication No. 51-12451, respectively. In addition, Japanese Examined Patent Publication No. 57-32716, Japanese Unexamined Patent Publication No. 55-89422, and Japanese Unexamined Patent Publication No. 56-75577 disclose that an Sr compound is effective for enhancing the properties of a forsterite film. It is to be noted that Japanese Unexamined Patent Publication No. 56-75577 allegedly eliminates, by means of a Sr compound, a defect of the film which incidentally arises due to the addition of a sulfide, such as Fe.sub.2 S, as proposed in Japanese Unexamined Patent Publication No. 53-50008, for stabilizing the secondary recrystallization. It is difficult to satisfy both the material properties and the interfacial properties, as can be understood from Japanese Unexamined Patent Publication No. 56-75577.
The development of additives for an annealing separator has been directed toward the stabilizing of the secondary recrystallization and the formation of a forsterite film, as described above, but does not necessarily attain the optimum properties. For example, in Japanese Examined Patent Publication No. 54-14568, the chromium nitride-, vanadium nitride-, and titanium nitride-additives relieve the nitrogen at a temperature influenced by the oxygen partial pressure, e.g., the dew point of the annealing atmosphere, but usually 900.degree. C. or higher. This temperature lies in the proximity of the secondary recrystallization temperature. The steel can occasionally have a starting temperature for the secondary recrystallization lower than the nitrogen dissociation temperature. In this case, the secondary recrystallization may not be satisfactorily stabilized. Furthermore, the annealing separator should be improved in the light of forming an excellent forsterite film. As is known, the smaller the forsterite crystal grains constituting a forsterite film, the better become the mechanical properties, such as the adhesive property of a film. The TiO.sub.2 addition disclosed in Japanese Examined Patent Publication No. 51-12451 is effective for promoting the solid state reaction of MgO-SiO.sub.2 and the sintering of forsterite particles. Nevertheless, the grain size of a forsterite film obtained only by the addition of TiO.sub.2 is approximately 1.0 .mu.m and is not considered satisfactory. Subsequently, a method was disclosed in Japanese Unexamined Patent Publication No. 54-66935, in which the moisture and CaO quantities in the MgO powder are appropriately controlled, thereby obtaining a forsterite film having a fine average grain-size and an improved adhesive property. The forsterite particle size obtained by this method is 0.7 .mu.m or less and is not necessarily satisfactory. Accordingly, the additive, which is effective for the MgO-SiO.sub.2 solid state reaction, must be further developed. In addition, most of the additives developed to date, are strongly effective for only the secondary recrystallization or the forsterite film. Accordingly, one additive effective for the secondary recrystallization and another additive effective for the forsterite film must to be added, in a complex form, to the annealing separator. This is disclosed in Japanese Unexamined Patent Publication No. 53-50008 and Japanese Unexamined Patent Publication No. 56-75577. When the complex additives are used, a complicated operation is necessary for forming the magnesia into a slurry and thus the costs are increased.